The ANSS event ID is tx2024sxrb and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/tx2024sxrb/executive.
2024/09/26 02:22:07 31.558 -104.093 6.3 3.7 Texas
USGS/SLU Moment Tensor Solution ENS 2024/09/26 02:22:07:0 31.56 -104.09 6.3 3.7 Texas Stations used: 4O.BP01 4O.CT01 4O.CT02 4O.CV01 4O.DB02 4O.DB04 4O.LWM1 4O.LWM2 4O.MID01 4O.MID02 4O.MID03 4O.PL01 4O.SA04 4O.SA07 4O.WB02 4O.WB03 4O.WB04 4O.WB05 4O.WB06 4O.WB07 4O.WB08 4O.WB09 4O.WB10 4O.WB11 4O.WB12 4O.WW01 4T.NM01 4T.NM02 4T.NM03 TX.ALPN TX.MB07 TX.MB08 TX.MB18 TX.MB25 TX.MNHN TX.ODSA TX.PB01 TX.PB03 TX.PB05 TX.PB06 TX.PB07 TX.PB09 TX.PB10 TX.PB11 TX.PB12 TX.PB13 TX.PB16 TX.PB18 TX.PB19 TX.PB21 TX.PB23 TX.PB28 TX.PB29 TX.PB30 TX.PB31 TX.PB34 TX.PB35 TX.PB37 TX.PB38 TX.PB39 TX.PB40 TX.PB43 TX.PB44 TX.PB46 TX.PB47 TX.PB51 TX.PB54 TX.PCOS TX.PECS TX.VHRN Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.15 n 3 Best Fitting Double Couple Mo = 2.09e+21 dyne-cm Mw = 3.48 Z = 7 km Plane Strike Dip Rake NP1 135 50 -75 NP2 292 42 -107 Principal Axes: Axis Value Plunge Azimuth T 2.09e+21 4 214 N 0.00e+00 11 305 P -2.09e+21 78 106 Moment Tensor: (dyne-cm) Component Value Mxx 1.41e+21 Mxy 9.94e+20 Mxz -2.02e+18 Myy 5.79e+20 Myz -4.94e+20 Mzz -1.99e+21 ############## ###################### ############################ ############################## --###--------------############### --#---------------------############ ####------------------------########## #####---------------------------######## ######----------------------------###### #######-----------------------------###### ########--------------- ------------#### #########-------------- P ------------#### ##########------------- -------------### ###########----------------------------# ############---------------------------# #############------------------------- ##############---------------------- ################------------------ # ##############------------ T ######################### ###################### ############## Global CMT Convention Moment Tensor: R T P -1.99e+21 -2.02e+18 4.94e+20 -2.02e+18 1.41e+21 -9.94e+20 4.94e+20 -9.94e+20 5.79e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240926022207/index.html |
STK = 135 DIP = 50 RAKE = -75 MW = 3.48 HS = 7.0
The NDK file is 20240926022207.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -30 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.15 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 165 90 -20 3.00 0.1904 WVFGRD96 2.0 160 80 -50 3.23 0.2382 WVFGRD96 3.0 165 90 -65 3.33 0.3444 WVFGRD96 4.0 310 55 -80 3.41 0.4493 WVFGRD96 5.0 310 50 -80 3.44 0.5187 WVFGRD96 6.0 130 45 -80 3.46 0.5429 WVFGRD96 7.0 135 50 -75 3.48 0.5460 WVFGRD96 8.0 130 50 -80 3.57 0.5395 WVFGRD96 9.0 130 50 -80 3.59 0.5174 WVFGRD96 10.0 130 50 -80 3.60 0.4840 WVFGRD96 11.0 140 55 -65 3.61 0.4471 WVFGRD96 12.0 145 60 -60 3.61 0.4116 WVFGRD96 13.0 145 65 -55 3.62 0.3788 WVFGRD96 14.0 5 60 40 3.61 0.3560 WVFGRD96 15.0 5 60 35 3.61 0.3360 WVFGRD96 16.0 5 60 35 3.62 0.3198 WVFGRD96 17.0 5 60 35 3.63 0.3085 WVFGRD96 18.0 5 55 35 3.64 0.3004 WVFGRD96 19.0 10 50 35 3.65 0.2957 WVFGRD96 20.0 15 50 45 3.67 0.2927 WVFGRD96 21.0 20 45 50 3.69 0.2927 WVFGRD96 22.0 20 45 50 3.70 0.2945 WVFGRD96 23.0 20 45 50 3.71 0.2948 WVFGRD96 24.0 30 40 55 3.72 0.2940 WVFGRD96 25.0 30 40 55 3.72 0.2915 WVFGRD96 26.0 20 50 60 3.72 0.2872 WVFGRD96 27.0 25 50 60 3.73 0.2850 WVFGRD96 28.0 25 50 60 3.73 0.2843 WVFGRD96 29.0 25 50 60 3.74 0.2828
The best solution is
WVFGRD96 7.0 135 50 -75 3.48 0.5460
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -30 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.15 n 3
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01 Model after 8 iterations ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00 6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00 13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00 19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00 0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00